Keira Knightley practised sex face in a mirror and showed the director on Skype.
She has revealed the amazing X-rated way she prepared for her racy new film role.
In A Dangerous Method she plays young psychiatric patient Sabina Spielrein, who gets a sexual thrill from being spanked.
She says: "It was a huge part of what Sabina was doing a lot of the time. So I asked psychoanalysts about it and they said, 'Sex and anything like that is trying to release energy, trying to release pent-up emotion'.
"So I worked with that and sat in my bathroom and pulled faces at myself for about two days, trying to figure out what it was going to be.
"Then I got on Skype with David and went, 'I've come up with this', or, 'I've come up with this', and he went, 'That one!' "
The film is based on the true story of Sabina, who formed a sado-masochistic relationship with her famous shrink Carl Jung, the 20th-century Swiss psychiatrist regarded as the founder of modern psychology.
Keira, 26, was keen to work with Cronenberg, director of such controversial films as 1996's Crash, which was about couples who are turned on by road accidents.
However, she was so concerned about the sex scenes — which include her being tied to a bed and spanked — that at first she decided to turn down the role.
She phoned Cronenberg to discuss the spanking scenes and recalls: "I went, 'I love you, I love it, but I really don't know if I want to do that. I'm really not sure about it'.
"And he went, 'Well, I think it would be a tragedy if you didn't do the part because of that so if you want them out then we just won't shoot them or we'll find a way that will make you comfortable doing it'.